'On two occasions I have been asked, 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question.' - Charles Babbage
whenever I come a cross a stubborn bolt/nut, I tighten it slightly before backing it out. It usually does the trick and if it happens to be a left thread that then reveals itself.......